In vertical wells it is frequently desirable to fracture from the well at various locations along the length of the well. In other words, a given well may penetrate various oil-bearing or other zones of interest and it may be desirable to fracture each of the oil-bearing or other zones of interest. Typically, the fractures cannot be done simultaneously for a variety of reasons.
Such wells may be cased or uncased through the formations of interest but, for purposes of simplicity, the typical practices will be discussed by reference to cased wells. The well is typically perforated through a first, and typically a lower, zone of interest. A tubing is then extended into the well to a depth above the first zone of interest and a packer is positioned to prevent the flow of fracturing fluid upwardly in the well between the outside of the tubing and the inside of the casing. A fracturing fluid is then injected into the well to fracture the formation through the perforations or, in the case of an uncased well, through a notched area of the formation of interest. After the fracturing has been completed, a sand plug is positioned over the fractured formation by filling the well with sand to a suitable level and thereafter a formation above the sand plug can be perforated and fractured by a similar technique. By the use of sand plugs of a variety of depths, a plurality of formations in the vertical well can be fractured independently of the other fractured zones. Typically, each zone is perforated separately so that the sand plug effectively isolates all the zones below the zone being perforated. Zones above the zone being perforated are typically perforated subsequently or are isolated from the zone being perforated by the packer.
In horizontal wells, by contrast, sand plugs are not readily usable because the sand slumps and exposes the fractures in the previously fractured zone, thereby exposing the previously fractured zones downstream from the packer to the pressure imposed to fracture at a second location upstream from the first fractured zone. The term "downstream" in this discussion is used to refer to the outer end of a horizontal section extending from a generally vertical section of a well with the term "upstream" being used to refer to locations in the well between the outer end of the horizontal section of the well and the end of the horizontal section at its junction with the vertical section of the well.
Typically, pairs of packers have been used to isolate a zone to be fractured in the horizontal section of the well. The packers are carried into the well on a tubing or other suitable tool string with the first packer being set downstream of the fracture zone and the second packer being set upstream of the packer zone with fracturing fluid thereafter being injected into a fracture zone between the two packers to fracture the horizontal well at the desired location. A plurality of zones in the horizontal section can readily be fractured separately using this technique, but it is a relatively expensive and complicated technique.
Accordingly, since it is desirable in many instances to fracture horizontal wells/sections at a plurality of locations along the length of the horizontal portion of the well, improved and more efficient methods have been sought for this purpose.